1. Field of the Invention
The present invention relates to a miniature acoustic transducer. More particularly, the present invention relates to a miniature acoustic transducer having a structure with a low spring constant.
2. Description of Related Art
The acoustic transducer, produced by a capacitive microphone chip integrated through silicon micro-manufacturing technique and integrated circuit (IC) processing technique, has the advantages of a light mass, a small volume and a good signal quality etc. In applications of national home appliances products, as the demand for handsets has expanded increasingly and the requirements for sound quality have enhanced increasingly, and the markets and techniques for hearing aids have started to flourish as well, capacitive microphone chip has gradually become a mainstream of microphone chips. From the perspective of the market, it is anticipated that the North American market of microphone chips will reach the level of 500 millions in the year of 2004, and will grow stably by 20% annually towards the market from 2004 to 2009, according to the sections about mobile handsets in the market trend reports by Digitimes Corp. Application of microphones on handsets becomes the mainstream of the present market.
Because integrated circuit processes using silicon as the base material are cheap and frequently employed in electronic products, and their application field continues to expand outward, more applications will be fabricated through processes using silicon as the base material combined with the CMOS process to directly integrate reader circuits onto a chip in the future. Additionally, since Taiwan has become the globally largest contract manufacturer for semiconductors, with a contract manufacture share of about 60-70% in the current market, mass production and acceleration of its commercialization process are expected in the future. Therefore, in order to keep away and differentiate in terms of the microphone layout from element designs by various primary factories, it is necessary to acquire novel designs and seize the first chance in manufacturing in the first place, so as to obtain the superiority in the microphone element market and the capability to share the occupancy.
Presently, the application of microphone element structures in mass production is limited to a few types of structures, because manufactories producing micro electro-mechanical systems (MEMS) microphones are currently only a few manufactories, such as Knowles Corp., Infineon Corp. or Sonion Corp., and most of the package processes on the market are still based on the designs developed by Knowles.
Referring to FIGS. 1 to 3, a microphone structure design by Knowles Corp. is shown. An acoustic transducer 10 includes a conductive diaphragm 12 and a perforated member 40, which are supported by a base 30 and separated by an air gap 20. An air gap 22, extremely thin, is present between the conductive diaphragm 12 and the base 30, to enable the diaphragm 12 to move up and down freely and decouple the diaphragm 12 from the base 30. A number of indentations 13 are formed beneath the diaphragm 12, for obviating adsorption phenomena between the diaphragm 12 and the base 30.
The lateral movement of the diaphragm 12 is restricted by the support portion 41 of the member 40, which may serve as a suitable enabling space between the diaphragm 12 and the member 40. Such support portion 41 may be constructed of a ring or a number of bumps. If the support portion 41 is constructed of a ring, a tense sound-sealed space would be formed when the diaphragm 12 rests against the support portion 41, and as a result, the acoustic transducer would have a well-controlled low frequency roll-off. A dielectric layer 31 is provided between the diaphragm 12 and the base 30. A conducting electrode 42 is fixed beneath the nonconductive member 40. The member 40 has several holes 21, and the diaphragm 12 also has several holes for creating a passageway 14 for sound flow with the holes 21 in the member 40.
The microphone structure design by Knowles Corp. is mainly a finger structure design directed to a back plate for increasing the strength of the back plate so as to reduce the resistance of the back plate. The diaphragm utilizes a design approach of decreasing the residual stress, and employs a common circular diaphragm design. The diaphragm provides only a simple support, and although its structure can avoid the problem of residual stress and a high natural frequency response, the effective deformation amount and the compliance of its design are still inadequate.
Referring to FIG. 4, another microphone structure design by Knowles Corp. is shown. This structure is essentially the same as that of FIGS. 1 to 3, with the only difference that the diaphragm 12 is connected to the base 30 via several spring structures 11 in order to decrease the intrinsic stress of the diaphragm and the stress generated from the base 30 or the packaged device.
Traditional microphone element designs utilize a simple and fixed diaphragm design. Although there are design approaches for increasing the diaphragm compliance, such as the finger structure shown in FIG. 5 in which a diaphragm 510 has a finger structure, or the corrugated structure shown in FIG. 6 in which a diaphragm 610 has a corrugated structure, most designs have disadvantages. Though the diaphragm of finger diaphragm design is soft and sensitive, it has a low resonant frequency response and is prone to fracture. Though the diaphragm of corrugated diaphragm design can effectively reduce the influence of the residual stress to enable large diaphragm compliance, it has a complicated process and is difficult to be processed, and the increase in the compliance is limited.